Under normal conditions, Free Fatty Acids (FFAs) are implicated in numerous physiological processes by serving as fuel in various metabolic pathways and/or acting as signaling molecules in different tissues such as the heart, liver, skeletal muscle, adipocytes and the pancreas (Newsholme et al., Biochem. J., 80 pp 655-662, 1961; Prentki et al., Endocrine Reviews, PubMed print ahead, 2008). Among FFAs, the short-chain fatty acids (SCFAs, carbon length C2-C6) are generated during anaerobic bacterial fermentation of fiber in the gut (Sellin et al., News. Physiol. Sci., 14, pp 58-64, 1999). Long-chain fatty acids (LCFAs, carbon length C14-C24) are products of dietary intake from adipose tissues and liver (McArthur et al., J. Lipid. Res., 40, pp 1371-1383, 1999).
Obesity is an increasing, worldwide public health problem associated with devastating pathologies such as type 2 diabetes (T2D) and dyslipidemia (Wild et al., Diabetes Care 27, pp 1047-1053, 2004). Dyslipidemia is characterized by high levels of triglycerides and/or LDL (bad cholesterol) or low levels of HDL (good cholesterol). Dyslipidemia is a key independent risk factor for cardiovascular diseases. It has long been suggested that FFAs are implicated in the regulation and/or genesis of these diseases (Fraze et al., J. Clin. Endocrinol. Metab., 61, pp 807-811, 1985). It is well established that regular intake of dietary fiber has several beneficial metabolic effects such as lowering of plasma cholesterol and triglyceride levels (Anderson et al., J. Am. Coll. Nutr., 23, pp 5-17, 2004). Specifically, dietary fiber has been shown to increase endogenous levels of SCFAs, leading to the suppression of cholesterol synthesis and improvement in glucose tolerance in rat (Berggren et al., Br. J. Nutr., 76, pp 287-294, 1996), as well as the reduction of hyperglycemia in a diabetic mice model (Sakakibara et al., Biochem. Biophys. Res. Com., 344, pp 597-604, 2006).
Drug therapies are available to address both T2D and dyslipidemia. Specifically, statins, fibrates and nicotinic acid or combinations thereof are often considered as a first line therapy in dyslipidemia whereas metformin, sulphonylureas and thiazolidinediones are three, widely-used classes of oral anti-diabetic drugs (Tenenbaum et al., Cardiovascular Diabetology, 5, pp 20-23, 2006). Although theses therapies are widespread in their use, the common appearance of adverse effects or lack of efficacy after long-term use causes concern. Moreover, the growing patient population suffering from T2D, dyslipidemia and associated metabolic diseases creates a demand for new entrants into this therapeutic market.
GPR43 (also named FFA2R) belongs to a subfamily of G-Protein-Coupled Receptors (GPCRs), including GPR40 and GPR41 that have been identified as receptor for FFAs (Le Poul et al., J. Biol. Chem. 278, 25481-489, 2003; Covington et al., Biochemical Society transaction 34, 770-773, 2006). The 3 family members share 30 to 40% sequence identity with specificity toward different fatty acids carbon chain lengths, with SCFAs (short chain fatty acids: six carbons molecules or shorter) activating GPR41 and GPR43; and medium and long chain fatty acids (MCFA, LCFA) activating GPR40 (Rayasam et al., Expert Opinion on therapeutic targets, 11 661-671, 2007). C2 acetate and C3 propionate are the most potent activators of GPR43. GPR43 is mainly coupled with Gq-proteins, with some evidence for its possible coupling with Gi/o pathways as well.
GPR43 is strongly expressed in adipocytes. Also there is evidence suggesting that GPR43 is overexpressed in pancreatic β-cells in prediabetic states as shown in WO2006/036688A2. Recent papers confirmed the GPR43 expression in pancreatic islets (Ahrén, Nature Reviews, 8 pp 396-385; 2009; Regard et al., J; Clin. Invst., 117 pp 4034-4043, 2007). In adipocyte cells, GPR43 is induced during the differentiation process and increased during the high fat feeding in rodents, suggesting that GPR43 may affect adipocyte functions (Hong et al., Endocrinology, 146 pp 5092-5099, 2005). Indeed, it has been reported that acetate and propionate may stimulate adipogenesis via GPR43. In addition siRNA results hinted that acetate and propionate may inhibit lipolysis in adipocytes via GPR43 activation (Hong et al., Endocrinology, 146 pp 5092-5099, 2005). It is interesting to note that the effect of acetate on reducing plasma free fatty acids level has been documented in humans (Suokas et al., Alcoholism, clinical and experimental research, 12 pp 52-58, 1988; Laurent et al., European journal of clinical nutrition, 49 pp 484-491, 1995). In addition, it has been shown that (i) adipocytes treated with GPR43 endogenous SCFA ligands exhibit a reduction in lipolytic activity and such inhibition of lypolysis is the result of GPR43 activation and (ii) GPR43 activation by acetate results in the reduction of plasma free fatty acids level in vivo (Ge et al., Endocrinology, 149 pp 4519-26, 2008). Recently two GPR43 positive allosteric modulator molecules have been shown able to inhibit the lipolysis in adipocytes similarly to that of GPR43 endogenous SCFA ligands (Lee et al., Mol Pharmacol, 74(6) pp 1599-1609, 2008). Such results suggest a potential role of GPR43 in regulating plasma lipid profiles and aspects of metabolic syndrome.
On this basis, new agonists or partial agonists of GPR43 may be of therapeutic value for T2D mellitus and conditions that are associated with this disease including, lipid disorders such as dyslipidemia, hypertension, obesity, atherosclerosis and its sequelae.